Home Depot offers the Kidde Smoke and Carbon Monoxide Alarm, model no. KN-COSM-BA, for $24.48. (Amazon charges the same.) Opt for in-store pickup to avoid the $5.99 shipping fee. That's the lowest price we could find by $12. It features a low battery warning, test reset button, and Smart Hush, which temporarily silences nuisance alarms.
A close price: Walmart has it for $24.97.